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'Exceptionally rare' meteorite impact crater found in the Twin Cities metro

Had it struck in today's world, much of the Twin Cities would've been wiped out.

• JOE NELSON
• SEP 29, 2022

Tiny, fractured grains of sand called shocked quartz—pictured above under a microscope—are common identifiers of meteorite impacts. – Photo credit: Julia Steenberg

 

 

 

 

An exceptionally rare meteorite impact crater has been discovered in Inver Grove Heights, Minnesota, and the size of it suggests that had it struck today, it would've wiped out a massive chunk of the Twin Cities.

The crater was discovered several hundred feet underground by researchers with the University of Minnesota’s Minnesota Geological Survey (MGS). It's the first known impact crater in Minnesota and the 191st known crater in the world. 

The crater is approximately 2 miles across and covers 4 square miles, in addition to a larger area outside of the central crater that if linked – more studies are needed to confirm it – would make the size closer to 10 square miles, which is bigger than the town of Hutchinson (population 14,000) and about the same size as Fridley (population 27,700). 

College of Science and Engineering, Minnesota Geological Survey

While the discovery is a marvel from the cosmos, a giant rock speeding through Earth's atmosphere remains a terrifying threat to the planet. For example, in 1908 a meteor is believed to have exploded about five miles above the Earth's surface in a remote area of Siberia. 

 

The shockwave leveled more than 80 million trees over an area of about 830 square miles. That meteor, estimated to be about 50 meters in size, was smaller than the meteorite that hit Inver Grove Heights. 

Julia Steenberg, a geologist at the University of Minnesota who helped discover the impact crater, told Bring Me The News that calculations suggest the size of the meteorite that hit Inver Grove Heights was anywhere from 150-600 meters – the size of several football fields. 

"This would have been larger than [the Tunguska meteor] – wreaking havoc on the area for sure," Steenberg wrote in an email, noting that meteorites are known to impact Earth at about 17 kilometers per second, or 38,000 mph. 

In 2013, a meteor exploded in the sky over southwestern Russia. That one, estimated to be 17-20 meters in size, produced a shockwave that injured around 1,600 people, most of whom were cut by broken glass, according to NASA. 

NASA discovered that the 2013 meteor exploded 14 miles above the ground and released the energy equivalent of around 440,000 tons of TNT. The shockwave shattered windows over 200 square miles. The atomic bomb America dropped on Hiroshima, Japan in 1945 was the equivalent of 20,000 tons of TNT. 

 

That's a house-sized meteor with the power of about 22 atomic bombs. 

How the U of M discovery was made

The U of M research team made the discovery in Inver Grove Heights while looking at subsurface samples as part of a routine Dakota County mapping project. Typically, per Steenberg, the geology in that area consists of layers of sandstone, limestone and shale in flat, predictable stacks. 

But drilling samples from the crater area revealed bedrock that "appears to be severely disrupted," with layers "out of order" and in some places "turned completely upside down."

"The disruption is highly unusual, but by itself not necessarily proof of a meteorite impact crater," Steenberg explained. "However, more detailed examination of deep rock samples revealed a key piece of evidence, odd cloudy looking quartz sand grains that appeared to be fractured."

Those cloudy quartz sand grains turned out to be shocked quartz, which are known identifiers of meteorite impacts. Features within the shocked quartz found at the site "are only known to be produced by extreme shock compressions caused by meteorite impacts," Steenberg said. 

Overall, the size of the impact crater in Inver Grove Heights is somewhat small compared to the 190 other known impact sites around the world. The largest is an estimated 160 kilometers in Vredefort, South Africa. The one in Inver Grove Heights is comparable to craters discovered in Decorah, Iowa and Rock Elm, Wisconsin. 

"It's big no doubt, but there have been much larger ones in Earth's history," Steenberg said. "Meteor Crater in [Arizona] is 1.2 km across for comparison, so smaller but still seems incredibly large when you are standing next to it!"